1. Field of the Invention
The present invention relates to a slow release formulation for pharmaceutical or veterinary use.
2. Description of the prior art.
The use of slow release formulations (also known as controlled release or sustained release formulatitons) is well established in medicine and the value of slow release formulations is widely appreciated. Slow release formulations have the advantage that the active compound is released over a relatively long period so that the active compound is maintained in the blood stream for a longer time and at a more uniform concentration than would otherwise be the case. It is also known to formulate slow release preparations to release the active compound only when the preparation has reached a certain part of the digestive system.
Many different proposals have been put forward for slow release formulations. One proposal for a slow release formulation is put forward in British Pat. No. 1021924: in the process of this patent the medicament is admixed with a comminuted sustained release material to obtain a dry mixture which is subsequently pressed into tablets. The sustained release material is said to be advantageously used in amounts of as much as 95%, a very high proportion indeed.
It is also known to prepare slow release formulations by incorporating active ingredient in a water insoluble binder which will disperse only very slowly in the alimentary system. For example, British Pat. No. 1137379 discloses a multi-stage formulation process in which ethylcellulose (which is water insoluble) is used as binder in the initial step. It would appear that in GB 1137379 insufficient alcohol is used in the initial processing to dissolve the ethylcellulose and thus a matrix (i.e. a uniform, continuous phase) is not formed. The process disclosed is very complex and would not be economical.
Another controlled release formulation which utilizes a water insoluble polymer is U.S. Pat. No 3,962,414. The polymer of U.S. Pat. No. 3,962,414 is initially water soluble but is cross-linked with polyvalent metal cations in the final formulations, and the patent discloses three different structures which use the cross-linked polymer to release drugs to the eye.
British Pat. No. 1486288 describes a water insoluble matrix which holds an active substance (e.g. a drug). EP 0094513 covers a device having not a water soluble matrix but a biodegradable one. The device is suitable for use as an implant, because the release of active material has a duration of several months or more and is achieved by biodegradation of the polymers comprising the system.
U.S. Pat. No. 2,809,916 discloses a formulation process using repeated steps (processes using 9 to 15 steps are exemplified) of mixing a drug with water insoluble excipient, drying and granulating. At each granulation step the mix is granulated to the same size and the result of such a process is a uniform and intimate mixture of excipient and active ingredient. The reason for carrying out the multistage process is apparently that sufficient excipient could not be combined with the active ingredient in one step - if all the excipient were added in one step the result would be an unworkable slurry. The US patent states that by increasing the number of mixing-drying steps the rate of release of the drug is slowed but it is to be expected that when the amount of insoluble binder is increased in this way the rate of release will be slowed.
A two stage formulation process is also disclosed in Example 8 of U.S. Pat. No. 3,946,110, in which aspirin powder is first mixed with pectin, then granulated and mixed with potato starch and silica. The potato starch, however, acts not as a slow release binder but as a disintegrator to accelerate release of aspirin. The inclusion of silica is for absorption of atmospheric moisture before this has a chance to react with the aspirin to form acetic and salicylic acids.
I have now appreciated that there remains a need for an economical slow release formulation. In particular, it would be desirable to provide slow release formulations which need only include cheap and readily available excipients and which can be made using existing machinery.